Ion implanters have been used for many years in the processing of semiconductor wafers. Typically, a beam of ions of a required species is produced and directed at a wafer or other semiconductor substrate, so that ions become implanted under the surface of the wafer. Implantation is typically used for producing regions in the semiconductor wafer of altered conductivity state, by implantation of ions of required dopant into the wafer.
Known ion implanters include batch type implanters, of which one example is shown in U.S. Pat. No. 4,733,091 (assigned to Applied Materials, Inc.), and single wafer-type implanters, such as described in U.S. Pat. No. 5,229,615 (assigned to Eaton Corporation).
In typical batch type implanters, wafers being implanted are mechanically scanned in each of two generally orthogonal directions, repeatedly through a fixed direction ion beam, to ensure an even implantation dose over the entire wafer surface of each wafer. In typical single wafer-type implanters, the ion beam itself is scanned transversely in one orthogonal direction at a relatively high scanning rate, and the single wafer being implanted is mechanically translated to and fro across the scanned beam substantially in a second orthogonal direction.
In single wafer-type implanters, the ion beam can be scanned electrostatically or electromagnetically and it is normal practice to collimate the scanned beam so that the beam impinging on the wafer remains generally parallel to a desired beam direction during scanning.
To ensure correct implantation, a number of parameters must be controlled so as to ensure that the scanned beam covers the entire surface of the wafer being implanted. Chiefly, these are beam centering, beam parallelism, and alignment relative to the centre axis of the implanter. In order to do this, it is generally necessary to measure the ion beam and to use these measurements as a feedback for a scan controller.
Several approaches have been set out in the art to address the problem of beam scanning and/or collimation control. For example, in U.S. Pat. No. 4,494,005, assigned to Hitachi, Ltd., an ion implanter is shown wherein an ion beam is deflected by a magnetic field to scan in a fan shape across a rotary disc of wafers. Four equispaced, fixed photoelectric beam detectors are placed in front of the rotary disc and the signals obtained by these detectors are used as feedback control for a scanning velocity controller which adjusts the energization current of the magnet that generates the magnetic field for scanning.
In EP-A-0,975,004, and in EP-A-0,457,321 (Nissin Electric Co. Ltd.), two Faraday arrays are employed at respectively upstream and downstream positions to measure current density distributions of the ion beam in the transverse scanning direction at those positions. The current density distribution of the ion beam is then estimated by interpolation of the upstream and downstream data to produce an interpolated error at the wafer itself. This is used to control the beam scanning, to reduce the error.
U.S. Pat. No. 4,992,106, assigned to Varian Associates, Inc., employs a Faraday detector which is slowly translated in the same direction as the ion beam scan to obtain an integrated ion beam dose (i.e. a beam intensity) as a function of detector position. This is employed to adjust the waveform of a voltage applied to an electrostatic deflector which steers the ion beam, so as to cause the integrated beam intensity to be uniform throughout the scanning length.
U.S. patent application Ser. No. 09/686,803, assigned to Applied Materials, Inc., describes one particular implementation of a single wafer-type implanter. In this document, the contents of which are incorporated by reference in their entirety, an ion implanter is disclosed in which the ion beam is magnetically scanned to and fro relative to a wafer. A travelling Faraday is located downstream of the wafer and is employed to collect ions from the ion beam to generate a timing signal. This timing signal is in turn used as a feedback signal to the scan and collimator controllers which control beam centering, parallelism, and alignment relative to a centre axis of the implanter.
There is a general requirement, in the ion implantation process, to produce dopant concentrations of a precise magnitude and with a high degree of uniformity over the surface of the wafer being implanted. This must be done even in the presence of beam current fluctuations instrumental variations, and distortions due to ion optics. Thus it is necessary to control the dopant concentration during the course of each scan across the wafer, to obtain the desired dosing profile. Control of the scan waveform applied to the scan and/or collimator controllers is also necessary so as to govern the implantation over the whole wafer. The present invention addresses these requirements.